1. Field of the Invention
The present invention relates to an abnormality diagnosis apparatus for a secondary air supply assembly for an internal combustion engine, wherein the secondary air supply assembly supplies secondary air to an exhaust gas passage of the internal combustion engine in order to quickly warm up a catalytic converter.
2. Description of Related Art
For example, as known in JP-A-2003-83048 and JP-A-2004-11585, air pump supplies secondary air to an exhaust pipe of the internal combustion engine at a specific position in order to purify HC or CO in exhaust gas (oxidation reaction) or in order to facilitate the warming up of a catalytic converter by the heat of reaction. In the above, the specific position may be a position within the exhaust pipe upstream of the catalytic converter that purifies exhaust gas.
When the secondary air supply assembly has failure, quality of emission of exhaust gas deteriorates. Thus, in JP-A-2003-83048 and JP-A-2004-11585, a pressure sensor is provided to the secondary air supply pipe, and abnormality diagnosis of the secondary air supply assembly is executed based on pressure of secondary air detected by the pressure sensor.
However, cost is increased disadvantageously in the above configuration, because there is needed to provide the pressure sensor in the secondary air supply pipe.
Thus, as described in JP-A-2004-204715, a secondary air flow amount is computed based on an output of an air-fuel ratio sensor that is provided at a specific position within the exhaust pipe of the internal combustion engine. For example, the air-fuel ratio sensor is provided at a position within the exhaust pipe downstream of a secondary air supply hole, through which secondary air is supplied into the exhaust pipe. Then, abnormality diagnosis of the secondary air supply assembly is executed based on the secondary air flow amount.
For example, a recent electronically controlled internal combustion engine is operated under an air-fuel ratio feed-back control, where fuel injection quantity is feed-back corrected based on the output of the air-fuel ratio sensor such that an air-fuel ratio estimated based on exhaust gas becomes a target air-fuel ratio. However, the air-fuel ratio may vary due to the variation of performance of an air-fuel ratio control system (for example, variation of the output of the air-fuel ratio sensor caused by manufacturing variation or aging of the air-fuel ratio sensor, variation of a fuel injection quantity caused by manufacturing variation or aging of a fuel injection valve). Thus, the above variation of the air-fuel ratio control system is learned based on an air-fuel ratio feed-back correction amount, and the fuel injection quantity and the output of the air-fuel ratio sensor are corrected based on the learning result. Thereby it is possible to correct the variation of the air-fuel ratio caused by the variation of the air-fuel ratio control system.
In the above system, the learned value of variation of the air-fuel ratio control system is stored in a back-up RAM in general. Typically, the back-up RAM is a rewritable memory that keeps stored data using an on-board battery as a back-up power source while a power source of the control circuit is off. The learned value of variation of the air-fuel ratio control system is updated every time a predetermined learning execution condition is satisfied and the variation of the air-fuel ratio control system is learned. Thus, a learning accuracy (accuracy in the learned value) of the variation of the air-fuel ratio control system is improved, and thereby the accuracy in the correction of the variation of the air-fuel ratio is improved.
However, in a case immediately after the connection of the system with the on-board battery (for example, a case immediately after the manufacture of the vehicle, or after the replacement of the on-board battery), the learned value of variation of the air-fuel ratio control system has not been stored in the back-up RAM. As a result, the learned value of variation of the air-fuel ratio control system requires to be computed by updating an initial value. Thus, while a learning count (in other words, the number of times of update) of variation of the air-fuel ratio control system counted since a time, at which the ECU 30 is connected with the on-board battery, is substantially small, the learning accuracy of variation of the air-fuel ratio control system may be insufficient, and thereby variation of the air-fuel ratio caused by variation of the air-fuel ratio control system may not be accurately corrected. In the above case, the output of the air-fuel ratio sensor may include influence of variation of the air-fuel ratio control system. As a result, when abnormality diagnosis of the secondary air supply assembly using the output of the air-fuel ratio sensor is executed as described in JP-A-2004-204715, variation of the air-fuel ratio control system may influence the determination of abnormality of the secondary air supply assembly, and may cause erroneous determination.